A friction clutch typically includes two or more mating plates which rotate in conjunction with one shaft and a friction plate disposed alternately between the mating plates so as to rotate in conjunction with another shaft. When a piston is actuated so that the mating plate and the friction plate come into friction engagement with each other by pressing the mating plate, a torque may be transmitted between those two shafts. Mating plates and friction plates are soaked in lubricating oil so as to be cooled down by lubricating oil. Friction material is bonded on the surface (both surfaces) of the friction plate facing the mating plate. Oil grooves are usually provided on the surface of the friction material or between the adjacent friction material segments to make the lubricating oil flow.
When the piston is not operating in a friction clutch, namely the clutch is out of engagement (which is also out of operation and out of fastening), it is ideal that a torque is not transmitted at all between the mating plates and friction plates so as not to cause power loss. In practice, however, even in a disengagement state of a friction clutch, a small amount of torque is likely to be transmitted between the mating plates and friction plates because of the lubricating oil between them. This is occurred by a share resistance force of the lubricating oil which is generated by passing through the oil groove of the friction plate while lubricating oil is dragged in the circumferential direction by the mating plate which rotates in a disengagement state of a friction clutch. Torque transmitted in this way is referred to as drag torque.
It is known that the drag torque may be reduced by increasing the content of air in the lubricating oil. Also, in order to reduce the drag torque, friction plates provided with various-shaped friction materials and oil grooves have been devised so far, and some are disclosed in the following examples.